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We investigated a pattern of involuntary lower extremity stepping-like movements which recently appeared in a subject with a 17-year history of neurologically incomplete injury to the cervical spinal cord. The movements were rhythmic, alternating and forceful, involved all muscles of the lower extremities and could be reliably evoked by lying the subject down (supine) and extending his hips. Once in this position, the movements continued spontaneously, in the absence of external sensory perturbations, with a step-cycle duration of approximately 3.5 s (0.3 Hz). This rate could be either increased or temporarily halted by specific sensory inputs. Anaesthetizing the subject's right hip joint, in which we found evidence of pathology, led to a marked attenuation of the stepping movements for a period of approximately 15 min. We believe that a combination of (i) preserved but limited supraspinal tonic facilitation, and (ii) abnormal, perhaps noxious afferent inflow from the subject's right hip to the spinal cord may underlie the appearance of this highly unusual and involuntary movement pattern. The striking similarity between the movement and EMG patterns in this subject and those described in many reports using the surgically reduced cat model suggests that we were witnessing the first well-defined example of a central rhythm generator for stepping in the adult human.

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Previous reports have described signi®cant limitations in the daily use of functional neuromuscular stimulation (FNS) ambulation systems by persons with spinal cord injuries (SCI). The potential application of these devices to provide physiological bene®ts as an exercise modality has prompted a reconsideration of the technology. However, the acute physiological eects related to the use of FNS systems have not been thoroughly examined. The purpose of this study was to investigate the relationships of oxygen consumpation (VO 2 ), heart rate (HR), and ratings of perceived exertion (RPE) during FNS ambulation by persons with SCI paraplegia. Eleven persons with thoracic level paraplegia, aged 21.5 to 38.0 years, participated in an incremental FNS ambulation test. Metabolic measures were collected continuously via open circuit spirometry as the subjects performed a series of ambulation passes of progressively increasing pace. At the end of each ambulation pass, HR and RPE measures were collected. The test was terminated when either the subjects judged the eort to be maximal or when the investigators deemed the eort to be maximal based on HR. A strong linear relationship was documented between the VO 2 and HR measures of all subjects throughout subpeak levels of FNS ambulation. RPE did not vary proportionally with VO 2 until relatively high levels of exercise intensity were reached. This study indicates that HR, but not RPE, is an appropriate indicator of exercise intensity for persons with SCI paraplegia using a FNS ambulation system.

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